1. Field of the Invention
The present invention is concerned with water cooling towers equipped with pultruded structural components and assemblies, typically as a part of the tower as frames and supports. The structural components hereof can be used either in new tower constructions or in retrofits of existing wooden-frame towers. More particularly, the invention is concerned with towers employing pultruded FRP (fiberglass reinforced plastic) structural channels of improved design having increased strength characteristics with lowered material usage, and the combination of such structural channels with elongated tubular pultruded FRP beams to form improved structural assemblies.
2. Description of the Prior Art
In recent years, standard FRP structural pultrusions (e.g., tubular beams and channels) have become available from a number of suppliers and have been used in the construction of large industrial cooling towers. FRP offers superior corrosion and fire resistance properties compared to wood for some applications. Channel shapes are usually used for horizontal beams or girt members, whereas square tubes are commonly used for columns and diagonal strut members. These types of members are employed to build frames in the longitudinal and transverse directions of the cooling tower. Channel sections may also be used for intermediate joists to support fan decking, fill structure, or drift eliminators between principal frames. Self-drilling, self-tapping screws are often used for attachment of these joists to the frames. General constructions of this type are disclosed in U.S. Pat. No. 5,811,035. Tower framing is often built manually one member at a time and is referred to as a "stick built" construction. Alternately, frames known as "bents" can be assembled on the ground and hoisted into position.
Custom pultruded tower framing members have been proposed in the past. For example, U.S. Pat. Nos. 5,028,357, 5,155,961 and 5,236,625 disclose methods of erecting panels and beams that frame into pairs of flanges to give a cruciform (tic-tac-toe) column. U.S. Pat. No. 5,487,849 describes a modular cooling tower concept using large custom pultruded sidewall panels, custom corner moldings, and hollow beams used to carry water. U.S. Pat. No. 5,047,197 also describes pultruded structural members interconnected by couplers including a metal or synthetic resin sleeve that extends through a tubular beam and abuts the adjacent face of channel members; a bolt and nut assembly is passed through the sleeve for connection purposes. A problem with this construction is that the sleeve is configured to have a substantially larger inner diameter, as compared with the bolt; this means that the bolt cannot participate with the sleeve in resisting shearing forces. Moreover, this reference does not describe matching the bearing capacity of a sleeve through the channels with the sleeve through the tube. Rather, this reference promotes the use of blind rivets to connect structural members together, and is therefore not concerned with bearing capacities of bolted fasteners.
In addition to new tower constructions, FRP pultrusions are being increasingly used in reconstruction of wood-frame cooling towers. Serious wood rot in cooling towers is found occasionally and can often be the result of water chemistry and/or operational procedures which alternately wet out and dry the tower components repetitively. Rot is usually isolated in certain areas of the tower (e.g., in the warm moist plenum or the hot water basin regions). FRP components offer rot resistance and may be an economical choice for the tower owner if repeated replacement of wooden components is required. Unfortunately, adequate, economical, standard pultrusions are not currently available in compatible dimensions to replace wood members directly. For example, 4 inch and 6 inch deep channel members have exactly 4 inch and 6 inch depths. On the other hand, standard dimensional wooden 2.times.4s and 2.times.6s are actually 1.5 inches.times.3.5 inches and 1.5 inches.times.5.5 inches respectively. The 0.5 inch difference may present difficulties when an attempt is made to use FRP members. Thus, in a replacement of a top girt supporting a water basin, FRP channels cannot be notched 0.5 inches, because this would severely detract from the bending capacity and other structural features of the channels. Therefore, in the past it has been necessary to raise the basin floor 0.5 inches to accommodate the FRP channels. This in turn leads to modification in the piping details associated with the basin.
Standard structural pultruded cross-sections owe their origins to the steel industry. The pultrusion industry mimicked the steel shapes due to the familiarity of the design professionals with steel construction. Available pultruded shapes are typically, I-beams, wide flange beams, channel shapes, square and round tubes, and angle shapes. The variability of available shape sizes in pultrusion is not as vast as in structural steel. Therefore, selection of an adequate section for a member may result in substantial capacity above what a design requires. This results in inefficient use of material and lessens the economy of pultrusions. The material and structural properties of pultrusions are very different from steel. Thus, it is only natural for pultruded shapes to evolve into different forms which optimize their characteristics.
Pultrusions generally have nonhomogeneous, orthotropic material properties due to unidirectional reinforcements oriented in the lengthwise direction (the direction of the pull). Continuous strand mats are often used to provide some reinforcement in the crosswise direction (normal to the direction of pull). Typical pultrusions have strong material properties in the lengthwise direction and are weaker in the crosswise direction. Beam shear is usually weak because of the weaker crosswise reinforcements.
Steel is generally classified as a homogeneous, isotropic material. Typical FRP and steel material properties are listed in the following table.
Table Comparing Material Properties of FRP Pultrusion and Steel Property Direction* Pultrusion Steel Tensile Strength, psi LW 30,000 36,000 CW 7,000 36,000 Tensile Modulus, psi LW 2,500,000 29,000,000 CW 800,000 29,000,000 Compressive Strength, psi LW 30,000 36,000 CW 15,000 36,000 Flexural Strength, psi LW 30,000 36,000 CW 10,000 36,000 Flexural Modulus, psi LW 1,800,000 29,000,000 CW 800,000 29,000,000 Beam Shear Strength, psi -- 4,500 17,000 Specific Gravity -- 1.7 7.8 *LW = Lengthwise; CW-Crosswise
Pultruded material strengths based on ultimate strengths. Steel material strengths based on yield strengths.
Thus, despite the widespread use of FRP pultruded structural members in cooling towers, significant problems remain which detract from the overall utility of the concept.